The present invention relates to improved kaolin clay products suitable for use in rotogravure printing processes, for coating lightweight and ultra lightweight paper for rotogravure and offset printing, and to methods for making the same.
Large amounts of kaolin find use as pigments in paper coating and filling compositions. Kaolinite, the principal constituent of kaolin clay (or kaolinitic clay), is a white clay mineral that imparts brightness, gloss, smoothness, printability, and other desirable properties to the surface of coated paper, paper board, super-calendared paper, and other paper related products.
Kaolin clays also find use as pigments in rotogravure paper, including lightweight coated paper (LWC) and ultra lightweight coated paper (ULWC), and as pigments and extenders in rotogravure printing inks. Rotogravure is a printing system commonly used to print large numbers of high quality images. Typical applications include the printing seen on packaging materials and in color magazines. In rotogravure printing, the image to be printed is etched into the surface of a printing apparatus, typically a metal roll or sheet. Ink is applied to the surface of the apparatus, filling holes or cells that comprise the etched image. Printing occurs when the surface to be printed (paper or plastic, for instance) directly contacts the printing apparatus.
Lightweight coated, or LWC, paper is generally used for printing magazines, catalogues and advertising or promotional material. LWC paper coated to a weight of from about 5 g/m2 to about 13 g/m2 on each side, and the total grammage, or weight per unit area of the coated paper, is generally in the range of from about 49 g/m2 to about 65 g/m2. The coating may conveniently be applied by means of a coating machine including a short dwell time coating head, which is a device in which a captive pond of coating composition under a slightly elevated pressure is held in contact with a moving paper web for a time in the range of from 0.0004 second to 0.01 second, before excess coating composition is removed by means of a trailing blade. However, other types of coating apparatus may also be used for preparing lightweight coated paper. The coated paper is required to meet certain standards of surface gloss and smoothness. For example, the paper is generally required to have a gloss value of at least about 32, and up to about 60, TAPPI units, and a Parker Print Surf value in the range of from about 0.5 to about 1.6 xcexcm.
Ultra lightweight coated, or ULWC, paper, sometimes otherwise known as light lightweight coated, or LLWC, paper is generally used for catalogues and for advertising and promotional material sent through the mail to reduce mailing costs. The coating weight is generally in the range of from 5 g/m2 to 7 g/m2 per side. The grammage is generally in the range of from about 35 g/m2 to about 48 g/m2.
Rotogravure, like other printing processes, typically requires kaolin pigments having good brightness, and which are capable of imparting good opacity, gloss, and smoothness to printed surfaces. The latter property impacts printability. The higher the smoothness, the better the printability.
To enhance smoothness and printability, it is most advantageous in rotogravure printing applications to use kaolins that allow the most efficient transfer of the ink from the printing apparatus to the surface being printed. When the ink from one or more cells of the etched image fails to transfer to the printed surface, a phenomenon called xe2x80x9cmissing dotsxe2x80x9d occurs. Fewer missing dots indicate better transfer and improved print quality. One method for quantifying ink transfer efficiency entails counting the distance between, for example, the first and the twentieth missing dot on a sufficiently large printed area. The longer the distance, the better the transfer efficiency and the better the quality of the printed image.
Kaolins most suited to rotogravure printing have a high shape factor (i.e., highly platy) and a controlled particle size distribution. Traditionally, kaolins mined from primary deposits, such as may be found in Cornwall, U.K., parts of Germany, and the Czech Republic, have found widespread use in rotogravure printing partly because their crystal structure allows them to be delaminated to give particles having an appropriate degree of platiness. More recently, it has been Found that certain sedimentary kaolin deposits, such as those found in Georgia, U.S.A., can also be delaminated to yield suitable high shape factor pigments. However, such kaolins are relatively impure and thus lack the degree of brightness and transfer efficiency that would be desirable in rotogravure pigments.
Kaolins from certain other sedimentary deposits, such as may be found in the Rio Capim region of Brazil, are known for their purity and brightness. But Rio Capim kaolins were generally thought to be too blocky to be of use in applications, like rotogravure printing, that require highly platy particles. Thus, Bilimoria et al., U.S. Pat. No. 6,312,511, disclose the use of Rio Capim clay as the source for both delaminated and non-delaminated kaolin pigments having high brightness and high opacity for paper coating and filling. The patent is silent, however, as to kaolins having high brightness and high shape factor, and does not disclose the use of Rio Capim crudes in the manufacture of kaolin pigments suited for various uses, including rotogravure printing. Willis et al., U.S. Pat. No. 5,169,443, teach the use of Rio Capim derived kaolin feed in the manufacture of pigments for rotogravure printing. Willis"" pigments are engineered to exhibit a specific combination of properties (viscosity, particle size distribution, and others) to render them highly workable at high solids concentration (67% solids and above). The patent is silent as to transfer efficiency, however, and does not disclose kaolins having high brightness and high shape factor. Thus, there remains a need in the art for improved kaolin pigments having higher brightness, improved transfer efficiency, and other properties. The present invention meets these and other needs.
The present invention provides kaolin clay pigments having at least the following characteristics:
Brightness (GE): at least about 89.0;
Shape Factor: at least about 30;
Brookfield viscosity, measured at 20 rpm and at less than or equal to about 65% solids: about 100 to about 700 centipoise; and
Hercules viscosity, measured at 18 dynes and at less than or equal to about 65% solids: about 200 rpm to about 1000 rpm.
The kaolins pigments of the present invention possess a combination of optical and physical properties, i.e., brightness and shape factor, not previously seen in existing commercial or known products. The present invention encompasses these pigments, compositions comprising them, and any application in which they are used. Such compositions include, but are not limited to, paper filler, paper coating, and ink compositions. Such applications include, but are not limited to, the corresponding uses of these compositions. In particular, coating, filler, and ink formulations comprising the inventive pigments may advantageously be used in gravure and rotogravure printing. When used in ink formulations and as paper fillers and coatings, the kaolin pigments of the present invention impart good printability and ink transfer efficiency.
The pigments of the present invention may be prepared according to the following process:
(a) Blunging and degritting crude kaolin;
(b) Classifying the blunged crude kaolin into coarse and fine fractions;
(c) Forming an aqueous suspension of the coarse fraction;
(d) Delaminating the aqueous suspension; and
(e) Beneficiating the aqueous suspension,
wherein the method results in a kaolin pigment having at least the following characteristics:
Brightness (GE): at least about 89.0;
Shape Factor: at least about 30;
Brookfield viscosity, measured at 20 rpm and at less than or equal to about 65% solids: about 100 to about 700 centipoise; and
Hercules viscosity, measured at 18 dynes and at less than or equal to about 65% solids: about 200 rpm to about 1000 rpm.